1. Field of the Invention
The present invention relates to a display screen equipped with control circuits for the screen pixels. More specifically, it relates to a liquid crystal display or LCD screens, to electroluminescent screens, to plasma screens, and to micropoint screens, also known as "field emission display" screens. Even more specifically, the invention relates to a process and an installation for assembling the control circuits for their subsequent connection to the screen.
2. Description of Related Art
A pixel screen is formed by a matrix of pixels controlled by means of a grid of perpendicular wires called a control grid. A screen of this type is also called a flat panel screen, as opposed to a curved screen in which the pixels are exited by a beam scanning the screen, such as a television screen.
Screen pixels are ordinarily controlled by a display card located outside the screen. The card is connected to the control grid by control circuits (drivers) formed by integrated circuits and ordinarily disposed on the rear periphery of the screen. The input-output terminals of the integrated circuits are connected to respective wires of the grid. Because of the large number of wires in the grid, some integrated circuits are disposed side by side along one edge of the rear side of the screen for the connection of the horizontal wires, and other integrated circuits are disposed in the same way on an adjacent edge for the connection of the vertical wires. The integrated circuits have input terminals which receive signals from the display card and output terminals connected to corresponding wires of the screen. The integrated circuits determine and control, based on signals they receive from the card, the state of the corresponding wires of the grid in order to control the associated pixels. In order for the space occupied by the integrated circuits on an edge to be minimal, their output terminals must have substantially the same pitch as the wires. Moreover, the integrated circuits must be separated from one another so as to be electrically insulated. In order to compensate for the space required for their insulation, it is necessary for the output terminals of the integrated circuits to be at a pitch smaller than the pitch of the screen wires.
The continually increasing miniaturization of pixels makes it possible to increase their density and to improve the quality of the image. At the present time, the pitch of grid wires can fall below 100 .mu.m, for example between 70 and 90 .mu.m. The pitch of the output terminals of the integrated circuits must be even smaller than the pitch of the wires. The problem under these conditions is to connect the grid wires to the output terminals of the integrated circuits by means of a process that provides good connection reliability, a practically null failure rate, and mass production at low cost.
One solution consists of alternating the side from which the wires of the grid are excited. Thus, one of every two wires is connected to an integrated circuit placed on one side of the screen, the other wire of each pair being connected to an integrated circuit placed on the opposite side. This makes it possible to divide the connection density in half. However, the edges of the screen must also carry other accessories, so at best, only one additional side can be occupied by the integrated circuits. Hence, the problem remains on one side.
Conceivable solutions to the problem include those for connecting the output terminals of an integrated circuit to the connection pads of the corresponding wires of the grid. The wire connection, according to the technology better known as "wire bonding," consists of soldering the end of a connecting wire to a pad, then to the corresponding output terminal, and of cutting the wire. Given the large number of connections of this type, this technology takes too long. A process adapted to low-cost mass production must make it possible to connect a group of output terminals to a corresponding group of pads (gang bonding).
From this point of view, a first conceivable solution consists of directly connecting the terminals of the integrated circuit to the connection pads of the grid wires. This arrangement places the integrated circuit in a position inverse to the one used ordinarily and is therefore called an inverted assembly, or "flip chip" assembly. The connection can be made by soldering or bonding by means of an electrically conductive adhesive. The extremely small pitch of the output terminals already poses a difficult problem in making this connection. But the major drawback of this solution is that it does not allow complete testing of the assembly of each integrated circuit, and consequently the screen is rejected if an integrated circuit is found to be defective. This is unacceptable.
A second conceivable solution is to mount the integrated circuit on a support according to the technology better known as TAB (Tape Automated Bonding). The support is ordinarily made of a flexible organic material provided with a central opening for housing an integrated circuit. One side of the support carries a group of leads having inner ends that extend into the opening and are disposed in correspondence with the respective terminals of an integrated circuit in order to be soldered to them. The soldering is an operation, commonly called an ILB (Inner Lead Bonding) operation, that can be carried out simultaneously on all or some of the terminals of the integrated circuit, either by means of balls (bumps) placed on the regions, or without balls (bumpless). The outer ends of the corresponding leads are then connected to the pads of the grid wires, by an anisotropic adhesive, for example. The conductors must therefore extend along the support at substantially the same pitch as the wires and have inner ends with a pitch smaller than that of the wires. At such a small pitch, the ILB operation poses a major problem. A grouped soldering of all the output terminals to the ends of the leads becomes impossible or can no longer be considered reliable.